Rotary engines such as the ones known as Wankel engines use the eccentric rotation of a piston to convert pressure into a rotating motion, instead of using reciprocating pistons. In these engines, the rotor includes a number of apex portions which remain in contact with a peripheral wall of the internal cavity of the engine throughout the rotational motion of the rotor.
One or more oil annular seals are typically provided in each end face of the rotor around the eccentric portion of the rotor shaft and are biased against the housing wall, to prevent oil from entering the combustion area. The oil is usually scavenged through an annular opening in the housing wall which must be sufficiently small to remain radially inwardly of the perimeter of the oil seals during rotation of the rotor. The load on the seal due to the oil pressure caused by the relative difficulty in evacuating the scavenged oil through the opening can become significant, increasing the risk of leaks.